The American National Standards Institute (ANSI) has defined certain standards for the performance of magnetic data storage tape drive apparatus. In particular, data to be recorded according to ANSI standards is recorded in blocks separated from one another by no less than 0.280 inches. This interblock gap is therefore that space in which a tape drive starting in the interblock gap must accelerate to its full design speed. Various tape velocities are standard, including up to 200 inches per second (ips). The goal of providing a tape drive which can accelerate to 125 ips within the 0.075 inch start distance (which is an acceleration of some 500 times the acceleration of gravity or 500 g's) is a requirement of a commercial tape drive.
The art has traditionally separated the mass of the tape carried on supply and take up reels from that portion of the tape in the vicinity of the read/write head (which is that portion of the tape which in fact must be accelerated to satisfy the requirements mentioned above) by decoupling that portion from the bulk of the tape, using vacuum columns in which loops of tape are carried. In this way, only the relatively small portion of the total tape in the vicinity of the head need be accelerated at very high rates, whereas the bulk of the tape carried on the reels can be accelerated relatively slowly. However, high acceleration of the portion of the tape in the vicinity of the heads is a significant engineering problem. To this end a vacuum capstan is typically used. Such a capstan is a hollow, cylindrical wheel having many holes in its periphery in contact with the tape. A vacuum is applied to the center of the hollow capstan so as to suck the tape into firm, non-sliding contact with the capstan. The capstan is made very light and of low inertia, so that it can be very quickly accelerated to speed, whereby the tape in contact with the capstan quickly reaches the design speed of the tape drive. See, for example, U.S. Pat. No. 4,065,044 to Painter et al for an example of such capstans.
The control of the acceleration of a capstan, even a relatively lightweight, low inertia one such as that disclosed in the Painter et al patent, remains a difficult problem. To this extent, numerous motion control schemes have been devised. According to the present invention, a microprocessor is used to continuously sample the actual speed of the tape drive and accelerate, decelerate or hold it at a steady angular velocity, as indicated by comparison of the actual velocity signal with a velocity command signal. The use of the microprocessor allows considerable versatility of the system to adapt to conditions encountered and allows long-term stability without regard to variations in, e.g., component tolerance and the like.